In diesel engines, pre-heater plugs are used to ensure that a certain temperature, known as the ignition temperature, is reached in the combustion chamber so that the combustion reaction of the air/fuel mixture can take place spontaneously when the pressure of the mixture is increased.
In order to pre-heat the air/fuel mixture it is known practice to use high-voltage pre-heater plugs which are powered with a dc voltage from the electrical voltage provided by the battery.
What a high-voltage pre-heater plug means is a plug which is powered at a voltage equal to the battery voltage, and what a low-voltage pre-heater plug means is a plug which is powered at a voltage lower than the battery voltage.
Nowadays, it is preferable to use low-voltage pre-heater plugs which do not take as long as high-voltage pre-heater plugs to reach the ignition temperature. This is because, during a rapid pre-heating phase, the low-voltage plugs will be powered at an overvoltage (at 11 volts) causing the temperature of the plug to increase very rapidly.
However, the duration of the rapid pre-heating phase needs to be controlled in order to avoid overheating that could lead to damage to the plug.
Thus it is found that restarting the engine, just after the engine has been stopped, causes a significant overheating of the tip of the plug, damaging it. The solution is therefore to wait for the plug temperature to have dropped sufficiently.
It is also known practice to use a temperature sensor to determine the temperature of the pre-heater plug and thus regulate its supply of power as a function of the temperature value acquired by the sensor, so as not to overheat the plug. However, use of such a sensor represents a high cost.